Clinical and epidemiological studies document a substantial overlap of at least 30 percent between attention deficit hyperactivity disorder (ADHD) and major depressive disorder (MD). Moreover, children comorbid for ADHD and MD are at heightened risk for morbidity and dysfunction. Clearly the ability to identify early the children most at risk for this comorbid presentation would be of the highest clinical and public health importance. To study early risk factors, it is necessary to choose a population known to be at especially high risk. Since parental MD has been shown to increase a child's risk for both MD and ADHD, and familial ADHD has been shown to increase risk for both MD and ADHD, a useful approach to studying early risk factors for these disorders in non-clinical samples is to focus on children at risk for MD and/or ADHD. Our work and that of others indicate that the temperamental trait "Behavioral Disinhibition" (BD) may be a powerful predictor of ADHD and externalizing symptoms and specifically of comorbid ADHD+MD. BD, which can be measured via laboratory observations in children ages 2 to 5 yrs, reflects the extreme tendency to seek out novelty, approach unfamiliar stimuli, and display disinhibition of speech and action. These responses may indicate a predisposition to develop ADHD+MD that can be measured before the disorders can be assessed accurately with standard psychiatric instruments. We therefore propose a 5-year longitudinal study of laboratory-observed BD in 2-5-year-old children and its psychiatric and functional outcomes (at age 6) in 100 offspring of parents with MD and 100 offspring of parents with ADHD, compared with 100 children of parents with neither disorder. We propose to examine BD as a predictor of (a) ADHD and (b) comorbid ADHD+MD. To examine the specificity of BD for these conditions, we will also examine rates of the broader categories of persistent disruptive behavior disorders, including oppositional-defiant disorder and conduct disorder, and of comorbid behavior disorders+MD. This study will form the foundation of a long-term follow-up study aimed at testing the usefulness of BD as a predictor of ADHD, comorbid ADHD+MD and of persistent disruptive behavior disorders in children at risk growing-up into later childhood and adolescence.